A Farewell to the Encephalization Quotient: A New Brain Size Measure for Comparative Primate Cognition

Abstract

Both absolute and relative brain sizes vary greatly among and within the major vertebrate lineages. Scientists have long debated how larger brains in primates and hominins translate into greater cognitive performance, and in particular how to control for the relationship between the noncognitive functions of the brain and body size. One solution to this problem is to establish the slope of cognitive equivalence, i.e., the line connecting organisms with an identical bauplan but different body sizes. The original approach to estimate this slope through intraspecific regressions was abandoned after it became clear that it generated slopes that were too low by an unknown margin due to estimation error. Here, we revisit this method. We control for the error problem by focusing on highly dimorphic primate species with large sample sizes and fitting a line through the mean values for adult females and males. We obtain the best estimate for the slope of circa 0.27, a value much lower than those constructed using all mammal species and close to the value expected based on the genetic correlation between brain size and body size. We also find that the estimate of cognitive brain size based on cognitive equivalence fits empirical cognitive studies better than the encephalization quotient, which should therefore be avoided in future studies on primates and presumably mammals and birds in general. The use of residuals from the line of cognitive equivalence may change conclusions concerning the cognitive abilities of extant and extinct primate species, including hominins.

The neuroecology of the water-to-land transition and the evolution of the vertebrate brain

“…B 377: 20200523 potentially increased computational power has been a perennial issue. While Jerison's measure of relative brain size, the 'encephalization quotient' [73], successfully identifies those species with relatively large brains for their bodies, its conceptual basis is problematic [91]. Encephalization measures very often do no better than absolute brain mass in predicting behavioural complexity, as in the cognitive control experiments described earlier [83].…”

Relative Brain Size and Cognitive Equivalence in Fishes

“…A second explanation is based on statistical logic: the estimated slope is reduced when the error in body size measures is greater than that for brain size [see detailed discussion in van Schaik et al, 2021]. This effect is strongly affected by the body size range in the sample and thus far greater within species than between them.…”

“…However, Tsuboi et al [2018] showed that the error problem was unlikely to explain this difference fully. Indeed, a recent analysis that used sexually dimorphic primates to calculate intraspecific slopes using mean values for males and females, and so minimize the error problem, obtained only moderately steeper slopes of 0.25-0.3 [van Schaik et al, 2021]. Nevertheless, it remains unclear what value we should expect for the ectotherms for 2 reasons.…”

“…Regarding total brain size analyses, Jerison [1973] had proposed the encephalization quotient (called EQ), the ratio of a species' actual brain size to its predicted brain size based on its cladespecific brain-body regression line, to capture its cognitive performance relative to other species within the same clade. The use of EQ as a predictor for relative cognitive performance across species has been criticised largely because of the taxon-level effects reported for endothermic vertebrates [van Schaik et al, 2021]. Indeed, EQ is a poor predictor of cognitive performance in mammals, with absolute brain size being a better predictor [Rensch, 1973;Deaner et al, 2007].…”

“…However, the use of absolute brain size ignores evidence that larger bodies warrant larger brains for body maintenance [Martin and Harvey, 1985]. Therefore, intraspecific slopes have been proposed as an alternative predictor of cognitive performance across mammals [van Schaik et al, 2021]. The key idea of this alternative approach is that the intraspecific slope reflects the extra amount of brain tissue required to sustain the additional somatic functions in larger individuals because there are no changes in bauplan and sensory-motor abilities.…”